An application of a high-performance secondary battery requires estimating a charge power corresponding to a state of charge (SOC) of the secondary battery.
For example, in a hybrid electric vehicle (HEV) and an electric vehicle (EV), a vehicle controller continuously demands the latest information on a charge power of a secondary battery from a battery management system (BMS).
The Hybrid Pulse Power Characterization (HPPC) is the power calculation technology for secondary batteries, widely known in the art.
HPPC can be found in the Partnership for New Generation Vehicles (PNGV) Battery Test Manual (Rev. 3, February 2001) published by the Idaho National Engineering and Environment Laboratory of the U.S. Department of Energy.
HPPC estimates power of a secondary battery considering only operation design limits, Vmin, Vmax, regarding a voltage of the secondary battery. Accordingly, this method does not consider the design limits associated with the state of charge (z) and currents of the secondary battery.
A “state of charge” as used herein refers to a relative ratio of currently remaining capacity relative to the capacity of a fully-charged secondary battery. A “state of charge” is represented by parameters, SOC or z. Parameter, SOC, is used to represent a state of charge in percentages. Further, parameter, z, is used to represent a state of charge in numbers between 0 and 1.
HPPC models the voltage of a secondary battery simply by Equation 1 below.V=OCV(z)+R×I  <Equation 1>
Here, OCV(z) is the open circuit voltage (OCV) of the secondary battery, which corresponds to the state of charge of the secondary battery, and R is a constant that represents the resistance of the secondary battery.
OCV(z) may be determined by an SOC-OCV look-up table pre-defined through tests. That is, an OCV(z) value may be obtained by mapping the open circuit voltage corresponding to the state of charge in the look-up table.
FIG. 1 illustrates in detail the concept of determining charge power of a secondary battery using HPPC.
As illustrated in FIG. 1, the final voltage of a secondary battery, Vch, is measured as soon as the secondary battery with a state of charge, Zk, is charged with a constant current having the magnitude of Ich for a certain period of time (for example: 10 seconds). Here, the final voltage of a secondary battery, Vch, may vary depending on the magnitude of the charging current and charging time.
Next, the slope of I-V profile, Rch, is determined based on Equation 1, and by using Rch, the linear equation, V=OCV(zk)+Rch*I, regarding the I-V profile is determined. Next, extrapolation is applied to the determined equation to determine the current value corresponding to Vlimit, the charge upper limit voltage. In this manner, the maximum charging current, Imax,ch, is determined.
According to HPPC, when the maximum charging current, Imax,ch, is determined, the charge power, Pc, is determined by Equation 2 below.Pc=Vlimit×Imax,ch=Vlimit×[(Vlimit−OCV(zk))÷Rch]  <Equation 2>
However, HPPC does not set the operation design limits for the charging current. If the maximum charging current, Imax,ch, of the secondary battery determined by HPPC is greater than the charge upper limit current that can be actually output by the secondary battery, the charge power is determined to be greater than the performance of the secondary battery. In this case, the secondary battery may be charged under a condition that is excessive than it should be. Especially, in the case of a lithium secondary battery, overcharging may be a cause of battery explosion.
Accordingly, there is demand for a new charge power estimation technology in the related field, which can overcome the shortcomings of HPPC mentioned above.